This invention relates a method of controlling a vehicle transmission. ‘Vehicle transmission’ here refers to a transmission to be interposed between a prime mover of the vehicle and ground engaging means of the vehicle which provides a plurality of gear ratios by which the ground-engaging means can be driven by the prime mover. The transmission has been devised primarily, but not exclusively, for a material handling vehicle, such as, for example, a tractor, loader type or similar vehicle for industrial and/or agricultural use, in which the ground engaging means typically comprises wheels but could also comprise crawler tracks.
Vehicles such as tractors or loaders have conventionally used transmission which provide a range of forward gear ratios. With a range of such transitions that the vehicle needs to develop useful tractive effort over a wide range of speeds. For example, in bottom gear the vehicle may be designed to have a maximum speed of 1.5 kph and in top gear have a maximum speed of 65 kph, to permit the vehicle to operate at low speed whilst under load, but enable the vehicle to travel at an adequate speed when travelling along a road.
To provide a wide range of close ratios, it is known to provide a transmission having effectively a pair of gear boxes, a first gear box drivingly connected to the engine and providing a first range of ratios and a reverse gear, and a second gear box connected to the output of the first gear box and providing a plurality of selectable gear ranges. The output of the second gear box is then connected to the ground engaging means. Consequently, where the first gear box provides six gears and the second gear box provides four gear ranges, it will be apparent that there are 24 torque paths through the transmission. It would be apparent that there may be a number of different torque paths which provide the same or overlapping speed ranges. Ideally, the selected gear should use the lowest possible range gear provided by the second gearbox. Advantageously, gear selection is performed by an electronic control unit which controls a solenoid to supply fluid to power shift clutches or operates a solenoid to move the synchromesh clutches in response to a gear shift demand from the operator. However, a problem with the second gear box, when it uses synchromesh clutches, is that it is desirable to effect the range shift as quickly as possible. For example, when travelling up an incline, it is desirable that an up-shift is effected as quickly as possible, because a slow gear change can result in the vehicle slowing excessively or even rolling backward. This is because, unlike power shift gear boxes, synchromesh gear boxes do not permit an unbroken supply of torque during upshift as it is necessary for the intermeshing parts of the gears to synchronise. A fast gearshift will also improve operator ‘feel’, by presenting a more responsive transmission.